Yahia Djaoued

Study of Anatase to Rutile Phase Transition in Nanocrystalline Titania Films

Nanocrystalline titania films were prepared by a complexing agent-assisted sol-gel dip-coating process. The effect of acetylacetone, diethanolamine and polyethylene glycol on the structure of the heat-treated titania films was examined by Raman and FTIR spectroscopy and X-ray diffraction. The effect the complexing agents have on the anatase to rutile phase transition during the heat treatment process is studied. The understanding of this effect is expected to enhance our capacity to tailor the composition and morphology of films and thus their properties. The Raman and the infrared spectra of nanocrystalline titania films and the changes induced by the heat treatment were also investigated. We have also studied the size of the crystallites in TiO2 films and its dependence on the type of complexing agent used.

Low Temperature Sol-Gel Preparation of Nanocrystalline TiO2 Thin Films

TiO2 nanocrystalline thin films with varying degree of porosity have been prepared using a low temperature method. TiO2 films of the anatase form have been obtained by using a polyethylene glycol (PEG) modified sol-gel method. Densification and crystallization of the films was found to result from the thermal treatment of the dip coated films in boiling water. The films have been characterized by Raman, XRD, FTIR, AFM and optical methods. Highly transparent films with transmission in excess of 85% and porosity as high as 58% are formed predominantly of anatase crystallites of dimensions of the order of 5 nm. Initial results on lithium intercalation into these films resulting in an efficient optical modulation in the visible and near infrared regions demonstrate a good potential of these films for electrochromic applications.

Controlled Growth of WO 3 Nanostructures with Three Different Morphologies and Their Structural, Optical, and Photodecomposition Studies

Tungsten trioxide (WO3) nanostructures were synthesized by hydrothermal method using sodium tungstate (Na2WO4·2H2O) alone as starting material, and sodium tungstate in presence of ferrous ammonium sulfate [(NH4)2Fe(SO4)2·6H2O] or cobalt chloride (CoCl2·6H2O) as structure-directing agents. Orthorhombic WO3having a rectangular slab-like morphology was obtained when Na2WO4·2H2O was used alone. When ferrous ammonium sulfate and cobalt chloride were added to sodium tungstate, hexagonal WO3nanowire clusters and hexagonal WO3nanorods were obtained, respectively. The crystal structure and orientation of the synthesized products were studied by X-ray diffraction (XRD), micro-Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM), and their chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS). The optical properties of the synthesized products were verified by UV–Vis and photoluminescence studies. A photodegradation study on Procion Red MX 5B was also carried out, showing that the hexagonal WO3nanowire clusters had the highest photodegradation efficiency.

Porous orthorhombic tungsten oxide thin films: synthesis, characterization, and application in electrochromic and photochromic devices

Porous orthorhombic tungsten oxide (o-WO3) thin films, stabilized by nanocrystalline anatase TiO2, are obtained by a sol–gel based two stage dip coating method and subsequent annealing at 600 °C. An Organically Modified Silicate (ORMOSIL) based templating strategy is adopted to achieve porosity. An asymmetric electrochromic device is constructed based on this porous o-WO3 layer. The intercalation/deintercalation of lithium ions into/from the o-WO3 layer of the device as a function of applied coloration/bleaching voltages have been studied. XRD measurements show systematic changes in the lattice parameters associated with structural phase transitions from o-WO3 to tetragonal LixWO3 (t-LixWO3) and a tendency to form cubic LixWO3 (c-LixWO3). These phase transitions, induced by the Li ions, are reversible, and the specific phase obtained depends on the quantity of intercalated/deintercalated Li. Raman spectroscopy data show the formation of t-LixWO3 for an applied potential of 1.0 V and the tendency of the system to transform to c-LixWO3 for higher coloration potentials. Optical measurements show excellent contrasts between colored and bleached states. An alternate photochromic device was constructed by sensitizing the o-WO3 layer with a ruthenium based dye. The nanocrystalline anatase TiO2 in the o-WO3 layer has led to an enhanced photochromic optical transmittance contrast of ∼51% in the near IR region. The combination of the photochromic and electrochromic properties of the synthesised o-WO3 layer stabilized by nanocrystalline anatase TiO2 opens up new vista for its application in energysaving smart windows.

Sol-gel-prepared ITO films for electrochromic systems

Abstract Indium tin oxide (ITO) films have been prepared by the sol-gel method using both organic and inorganic precursors. A computer-controlled dip-coating unit is designed and fabricated in our laboratory for a precise control of the parameters during the dip-coating process. These films have been characterized by X-ray diffraction optical and electrical study and also by atomic absorption spectroscopy. The optimized coatings exhibit a sheet resistance of ara und 100 Ω/□ and an average visible solar transmission of around 85%. A five-layer electrochromic system using these ITO layers as transparent electrodes was fabricated and tested. The performance of the electrochromic system indicates the high potential of these films for such applications, especially for large area coaling.

Synthesis and Characterization of Macroporous Tungsten Oxide Films for Electrochromic Application

Macroporous tungsten oxide films have been prepared by combining a nonhydrolytic sol-gel method with a molecular assembly templating strategy. The material has been prepared by hydrolysis of an ethanolic solution of tungsten ethoxide in the presence of polyethylene glycol (PEG), followed by calcination of the dip-coated films. AFM images indicated that an important morphological diversity can be obtained by simply varying the amount of PEG in the coating solution and the conditions of the heat-treatment. The formation of nanostructures of controlled shapes and patterns (fibrils or striped phases) with relatively uniform channel spacings is accounted for by strong hydrogen bonding interactions between the PEG and the partially hydrolyzed tungsten oxide oligomers. XRD and FTIR data showed that PEG delays the crystallization of WO3. When compared to sol-gel prepared tungsten oxide fims prepared without PEG, the coloration efficiency of the macroporous films appears to be significantly improved especially in the near-infrared region.

Synthesis of hybrid silica materials with tunable pore structures and morphology and their application for heavy metal removal from drinking water

Porous silica materials S8, S12, S16, and SBA with controllable pore structures and morphology were synthesized by varying the type or alkyl chain length of the surfactant. Diverse amino-functionalized organic-inorganic hybrid porous materials were then prepared by post-grafting. Depending on the relation between the pore diameter of the porous silica materials and the size and content of the moiety to be grafted, the functionalized materials exhibited varying degrees of decline of structure properties, i.e. regular arrangement of pores, specific surface area, pore size, and pore volume. The hybrid silica materials have been employed as heavy metal ions adsorbents from simulated drinking water at room temperature. The results indicated that the diverse pore structures and different amino group densities influence the heavy metal ions adsorption of functionalized silicas significantly. The best adsorbent was found to be monoamino-functionalized silica S16-1N, which could effectively remove heavy metal Cd(II), Pb(II), Fe(III), as well as Mn(II). The good performance can be attributed to the accessibility of effective amino groups in the pores, as well as the suitable pore structure with high specific surface area of 728 m(2)/g and total pore volume of 0.34 cm(3)/g.

Study of the phase transition and the thermal nitridation of nanocrystalline sol–gel titania films

Abstract Nanocrystalline titania films were prepared by a complexing agent-assisted sol–gel method and converted to titanium nitride by a thermal nitridation process. The effect of acetylacetone (AcAc), diethanolamine (DEA) and acid catalysts (HCl and HNO 3 ) on the structure and morphology of the heat-treated titania films and on their nitridation products was examined by FTIR spectroscopy, X-ray diffraction (XRD) and atomic force microscopy (AFM). The carbothermal reduction of titania during the nitridation process with the formation of carboxynitrides has been considered. The results showed that the oxide to nitride transition strongly depends on the complexing agent used to prepare the titania films. The XRD results indicated the dependence of the lattice parameter of the nitridation product on the complexing agent or acid catalyst: AcAc and DEA lead to TiN x with a lattice parameter α close to the theoretical value, while with HCl the lattice parameter was found sensibly lower showing the presence of an oxynitride. These results are accounted for by the effect of complexing agents and acid catalysts on the size of both TiO 2 and TiN grains and the different reactivity of the anatase and rutile phases. The possibility of tailoring the composition and morphology of TiN films by using complexing agents is envisaged.

Hydrothermal synthesis of anatase nanocrystals with lattice and surface doping tungsten species

We have synthesized anatase nanocrystals of 8–15 nm containing lattice and surface doping tungsten through hydrothermal reactions. We used phosphotungstate–titania nanocomposites with various compositions as the reagents. The homogeneous distribution of tungsten in the reagent solids appears to strongly influence the synthesis results. X-Ray diffraction and Raman spectroscopy data showed that the samples contained brookite impurities when the tungsten content was low (up to W/(W + Ti) = 3.7%) but pure anatase was obtained when the tungsten content was high (up to W/(W + Ti) = 26%). The tungsten content in the anatase nanocrystals is reflected in the lattice parameters, the Ti(W)–O bond distances, the Raman peak positions and peak widths. Tungsten atoms are present as lattice doping and surface coating (wolframyl group). The relative proportion between these species changes with the tungsten content: at low tungsten-content, the lattice doping tungsten is the dominating species that expands the lattice. Around the critical concentration of W/(W + Ti) = 13%, the amount of surface wolframyl group increases while the lattice doping appears to have reached a saturation. A formation mechanism that explains most of the observations was proposed.

Optical and electrochemical properties of vanadium pentoxide gel thin films

Optical and electrochemical properties of vanadium pentoxide (V2O5) films prepared by sol‐gel method from organic and inorganic precursors were studied. The organic gel was obtained by hydrolysis and polycondensation of vanadium isopropoxide dissolved in isopropanol. The inorganic gel was prepared from aqueous solution of sodium vanadate. Lithium insertion in LiyV2O5 films (0<y≤2.0) was investigated by cyclic voltammetry and electrochemical potential spectroscopy, showing good reversibility up to y∼2.0 and structure transitions from α phase to e phase and δ phase in the range 0<y≤1.0. Highest values for chemical diffusion coefficient were about 3×10−10 cm2/s at y∼0.8, as determined from complex impedance measurements.